System and method for optimizing solar power conversion

ABSTRACT

A solar power system is provided for maximizing solar power conversion. The solar power system includes n power units connected in series and n-1 DC-DC converting units, and each of the n-1 DC-DC converting units is coupled to at least one of n solar power units. Each of the n-1 DC-DC converting units is configured to control the correspondingly connected solar power units to operate at a target current generation. The solar power system further includes a controlling unit coupled to the n-1 DC-DC converting units. The controlling unit monitors and compares the n currents generated by the n solar power units. Based on the current comparison, the controlling unit determines a series current and controls the n solar power units so that each of the generated photovoltaic currents is substantially equal to the determined series current.

CROSS-REFERENCE TO RELATED APPLICATION

The present disclosure claims priority to U.S. Provisional ApplicationNo. 61/553,125, filed on Oct. 28, 2011, which is herein incorporated byreference in its entirety.

BACKGROUND

Unless otherwise indicated herein, the materials described in thissection are not prior art to the claims in this application and are notadmitted to be prior art by inclusion in this section.

A photovoltaic (PV) array is a linked collection of solar panels. MostPV arrays use an inverter to convert direct current (DC) power producedby the solar panels into alternating current (AC) power. The panels in aPV array are usually connected in series, as strings, to obtain adesired voltage. A plurality of individual strings is then typicallyconnected in parallel to increase current production.

In conventional PV power generation systems, there is a tradeoff betweenlocal power generation and system efficiency. Conventional PV systems donot support local control of PV panels. As shown in FIG. 1, aconventional PV system 100 typically includes a plurality of PV panels102 connected electrically in series to form a PV string 104, and asingle large inverter 106 processes power from the entire PV seriesstring 104 for delivery to a load or a power grid 108. In thisarrangement, if one or more of PV panels 102 are shaded or otherwiseenvironmentally compromised, power production is reduced for the entirestring 104. PV panel manufacturers have added reverse diodes topartially mitigate electrical impacts of local shading, soilin, orsimilar problems.

As known to one of ordinary skill in the art, simultaneously operatingeach individual PV panel 102 of PV string 104 close to or at itspotential maximum power production level, termed a maximum power point(MPP), as enabled by on-going environmental conditions, has proven to behard, even impossible, to attain. It is well established that thisinability to operate individual PV panels 102 at their correspondingMPPs sacrifices power production. Power reduction can be 20% or more ifPV system 100 is subject to local shading, and can be of the order ofseveral percents even when PV system 100 is uniformly illuminated.

A known improvement to the conventional PV system 100 provides localpower processing on a per PV panel basis. For illustration purposes,FIGS. 2 and 3 show DC-DC versions 200 and 300 that provide series andparallel connected DC-DC converters 206 and 306, respectively. As shownin FIG. 2, DC-DC converters 206 are connected in series via theirrespective outputs, while each is also coupled to one of PV panels 208.As shown in FIG. 3, DC-DC converters 306 are connected in parallel viatheir respective outputs, while each is also coupled to one of PV panels308. A DC-AC version 400, shown in FIG. 4, provides a direct couplingbetween a DC-AC inverter 406 and a PV panel power 408, without any DC-DCconverters therebetween. Based on the literature, DC-AC version 400 hasonly been used commercially in a parallel arrangement (not shown) of theDC-AC inverters 406, although a series version (not shown) has beenreported.

In each of DC-DC versions 200 and 300, a local DC-DC converter 206, 306is connected to a respective PV panel 208 308, and to a DC-AC inverter210, 310, for the delivery of power to a grid 212, 312. The DC-DCversions 200 and 300 sacrifice efficiency, since power needs beprocessed twice between each PV panel 208, 308 and the grid 200, 300. InAC version 400, a PV system, which includes a large number of PV panels408, also needs to include a corresponding number of DC-AC inverters406, each of which processes a power generated by the correspondinglyconnected PV panel 406, to grid 412. Based on the configurations ofFIGS. 2-4, all of the generated power must be processed through localDC-DC converters 206, 306 and DC-AC inverters 406, which can lead toexcessive power losses.

SUMMARY

Disclosed herein are improved methods, systems, and devices foroptimizing solar power conversion.

In one embodiment, a solar power system includes a set of n power unitsconnected in series, wherein each of the n solar power units generates aDC photovoltaic current and produces a DC photovoltaic voltage. Thesolar power system includes a set of n-1 DC-DC converting units, each ofthe n-1 DC-DC converting units is coupled to at least one of the n solarpower units, and wherein each of the n-1 DC-DC converting units isconfigured to control the correspondingly connected solar power units tooperate at a target current generation. The solar power system furtherincludes a controlling unit coupled to the n-1 DC-DC converting units,wherein the controlling unit monitors and compares the n currentsgenerated by the n solar power units. Based on the current comparison,the controlling unit determines a series current and controls the nsolar power units, via the n-1 DC-DC converting units, so that each ofthe generated photovoltaic currents is substantially equal to thedetermined series current, and when the controlling unit determines thatone of the n solar power units generates a photovoltaic current that isless than a predetermined current threshold percentage of the determinedseries current, the controlling unit causes the correspondingly coupledDC-DC converting unit to operate as a current bypass unit.

In another embodiment, a method is provided for maximizing powergeneration in a power solar system, which includes a set of nseries-connected solar power units and a set of n-1 DC-DC convertingunits. Each of the n solar power units generates a DC photovoltaiccurrent and produces a DC photovoltaic voltage, and each of the n-1 ofDC-DC converting units is coupled to and controls at least one of the nsolar power units. The method includes monitoring and comparing the ncurrents generated by the n solar power units, and based on the currentcomparison, determining a series current for the n solar power units.The method further includes controlling the n solar power units, via thecorrespondingly coupled n-1 DC-DC converting units, so that each of thegenerated photovoltaic currents is substantially equal to the determinedseries current, and determining whether one of the generatedphotovoltaic currents is less than a predetermined current thresholdpercentage of the determined series current. Based on the currentthreshold percentage determination, the method further includes causingthe correspondingly coupled DC-DC converting unit to operate as acurrent bypass unit.

In another embodiment, a non-transitory computer readable storage mediumcontains instructions that cause a computing system to perform theabove-discussed method for optimizing solar power conversion.

In another embodiment, a solar power system includes a plurality ofsolar power units connected in series, each of the plurality solar powerunits generates a DC photovoltaic current and produces a DC photovoltaicvoltage, and a plurality of DC-DC converting units, each of theplurality DC-DC converting units is coupled to at least one of theplurality of solar power units. Each of the plurality of DC-DCconverting units is configured to control the correspondingly connectedsolar power units so that they operate within a predetermined powerrange. Each individual DC-DC converting unit is configured to determinewhether a correspondingly connected solar power unit generates acorresponding DC photovoltaic current that is outside of a predeterminedcurrent range. Based on the outside current range determination, eachindividual DC-DC converting unit is configured to shunt thecorrespondingly connected power unit while the other power units remainconnected in series.

In another embodiment, a solar power system includes a plurality ofsolar power units connected in series, each of the plurality solar powerunits generates a DC photovoltaic current and produces a DC photovoltaicvoltage, a DC-AC inverting unit for inverting a cumulative DCphotovoltaic voltage of the plurality of produced photovoltaic voltagesto an AC power signal for distribution to one or more AC loads, and aplurality of DC-DC converting units. Each of the plurality DC-DCconverting units is coupled to at least one of the plurality of solarpower units, and is configured to control the correspondingly connectedsolar power units so that they operate within a predetermined powerrange. The solar power system further includes a capacitor unitconnected in parallel to the plurality of DC-DC converting units.

These as well as other aspects, advantages, and alternatives will becomeapparent to those of ordinary skill in the art by reading the followingdetailed description, with reference where appropriate to theaccompanying drawings. Further, it should be understood that thedisclosure provided in this summary section and elsewhere in thisdocument is intended to discuss the invention by way of example only andnot by way of limitation.

BRIEF DESCRIPTION OF THE FIGURES

In the figures:

FIG. 1 is a schematic diagram of a known power conversion systemincluding a plurality of PV panels connected in series to a DC-ACinverting unit;

FIG. 2 is a schematic diagram of a known power conversion systemincluding n DC-DC converting units connected in series, each of theDC-DC converting units coupled to a PV panel;

FIG. 3 is a schematic diagram of a known power conversion systemincluding n DC-DC converting units connected in parallel, each of theDC-DC converting unit coupled to a PV panel;

FIG. 4 is a schematic diagram of a known power conversion systemincluding n isolated DC-AC inverters, each of the DC-AC inverterscoupling a PV panel to a load;

FIG. 5 is a schematic diagram of an exemplary embodiment of a powerconversion system including n PV panels connected in series and n-1DC-DC converting units connected in series;

FIG. 6 is a schematic diagram illustrating an exemplary embodiment of acontrol system for controlling power generated by N series-connected PVpanels;

FIG. 7 is a schematic diagram of an exemplary embodiment of a powerconversion system including n PV panels connected in series and n-1DC-DC converting units connected in parallel;

FIG. 8A is a schematic diagram of an exemplary embodiment of a powerconversion system including n PV panels connected in series and n-1DC-DC converting units connected in parallel to an independent DC bus;

FIG. 8B is a schematic diagram of an exemplary embodiment of a powerconversion system including n PV panels connected in series and n DC-DCconverting units connected in parallel to an independent DC bus;

FIG. 9 is a schematic diagram of an exemplary embodiment of a powerconversion system including n PV panels connected in series and n-1DC-DC converting units, with each DC-DC converting units connected to acouple of PV panels;

FIG. 10 is a schematic diagram of an exemplary embodiment of a powerconversion system, similar to that of FIG. 9, including a plurality ofPV panels connected in series and configured to enable differentialenergy transfer between PV panels;

FIG. 11 is a schematic diagram illustrating an exemplary embodiment of alocal controller unit for providing MPP tracking in a differential powerconversion system;

FIG. 12 is a flow chart illustrating an embodiment of a method forcontrolling and optimizing power generated by n series-connected PVpanels; and

FIG. 13 is a schematic diagram illustrating a conceptual partial view ofan example computer program associated with the method of FIG. 12.

DETAILED DESCRIPTION

In the following detailed description, reference is made to theaccompanying figures, which form a part hereof. In the figures, similarsymbols typically identify similar components, unless context dictatesotherwise. The illustrative embodiments described in the detaileddescription, figures, and claims are not meant to be limiting. Otherembodiments may be utilized, and other changes may be made, withoutdeparting from the spirit or scope of the subject matter presentedherein. It will be readily understood that the aspects of the presentdisclosure, as generally described herein, and illustrated in thefigures, can be arranged, substituted, combined, separated, and designedin a wide variety of different configurations, all of which areexplicitly contemplated herein.

As stated above, power conversion system 100, 200 and 300, shown inFIGS. 2-4, are substantially inefficient as all of the power generatedby PV panels 208, 308, 408 has to be processed by both thecorrespondingly coupled DC-DC converting units 206, 306 and DC-ACinverting units 406, respectively.

To overcome this inefficient power conversion, an embodiment of a powerconversion system includes a plurality of solar power units connected inseries, and a plurality of DC-DC converting units, each of which iscoupled to at least one of the plurality of solar power units. Duringoperation, MPP values of a set of substantially similar PV panels,connected in series and operating under similar environmentalconditions, may be substantially equal. That is, the differences betweensolar powers produced by the series-connected PV panels aresubstantially small (minor).

Recognizing that MPP values of the series-connected substantiallysimilar PV panels, operating under similar environmental conditions, maybe substantially equal, the above-introduced embodiment of a powerconversion system is configured to manage the minor power differences,so as to maximize an overall power generated by the series-connected PVpanels. This process of managing minor PV power differences, i.e.,differential power processing, is configured to enable MPP tracking oneach PV panel, and to process the generated power as little as possible,such as only once by the DC-AC inverting unit, thereby minimizing powerlosses due to power processing. As such, this management of minor powerdifferences may avoid minor MPP value mismatches, while sendingsubstantially all of the PV power generated by the series-connected PVpanels directly to a DC-AC inverting unit for output processing.

Now referring to FIG. 5, an embodiment 500 of a PV conversion systemconfigured to implement the above-discussed differential powerprocessing is illustrated. As shown, power conversion system includes“n” PV or solar energy sources or power units 504 connected in series toform a PV panel string 503, and “n-1” DC-DC converting units 506connected in series. Power conversion system further includes a DC-ACinverting unit 508 coupled to PV panel string 503, and to the n-1 DC-DCconverting units 506. Each of the n PV power units 504, which canprovide a DC photovoltaic output, may be a PV cell, a PV array, a PVpanel, or a string of panels. For the sake of simplicity, PV power units504 will hereafter be referred to as PV panels 504. In one embodiment,each of the n-1 DC-DC converting units 506 is coupled to one of n-1 PVpanels 504, thereby leaving one of the n PV panels 504 uncoupled. Asshown in FIG. 5, PV₁ is the uncoupled PV panel 504, whereas each ofPV₂-PV_(n) panels 504 is coupled to one of DC-DC₁, to DC-DC_(n-1),converting units 506, respectively.

Based on the configuration of FIG. 5, a controlling method or process,performed by a controlling unit 604, shown in FIG. 6, is configured tofacilitate and control a series current I_(pv) from PV panel string 503,so as to maximize the power output of the PV string 503 by operatingeach PV panel 504 at its MPP, and to minimize the total power flowingthrough the n-1 DC-DC converting units 506. As discussed above, each ofthe n-1 DC-DC converting units 506 is monitored and controlled bycontrolling unit 604 so that each of their respective n-1 PV panels 504operates at or tracks a determined power output, such as a determinedMPP value or target. In one embodiment, in order to achieve a global MPPtracking for PV panel string 503, DC-AC inverting unit 508 may beconfigured to monitor and control the uncoupled PV panel 504, e.g., PV₁504 of FIG. 5, to enable it to operate at or track its MPP value asdetermined by controlling unit 604. Alternatively, DC-AC inverting unit508 is configured to maximize the overall power output of PV panelstring 503 by also maximizing the power out of the uncoupled PV panel504. As such, controlling unit 604 is configured to control n MPPvalues, i.e., n MPP variables, via n controlling actuators, i.e., n-1DC-DC converting units 506 and one DC-AC inverting unit 508. Thisarrangement of controlling unit 604 enables power conversion system 502to reach an operating level that substantially maximizes its total poweroutput, which represents the total power generated by PV panels 504 lessthe power losses due to MPP tracking and power conversion.

During operation, while performing an MPP tracking locally, i.e., foreach PV panel 504 or performing an MPP tracking globally, i.e., for thewhole PV panel string 503, controlling unit 604 monitors and comparesthe n currents generated by the n PV panels 504, and based on thecurrent comparison determines the series current I_(pv). Moreover,controlling unit 604 controls n-1 PV panels 504, via their respectiven-1 DC-DC converting units 506 and AC-DC inverting unit 508,respectively, so that each of the generated photovoltaic currents issubstantially equal to the determined I_(pv) series current. As such,controller unit 604, via a differential power processing application610, is configured to ensure each of the generated photovoltaic currentsis substantially equal to the determined I_(pv) series current, so thatthe bulk of the generated photovoltaic power flows through theseries-connected PV panels 504, and only a small fraction or percentageof this photovoltaic generated power is processed by n-1 DC-DCconverting units 506, as needed to monitor and control power productionof each of the n-1 coupled PV panels 504 at or near their respectivedetermined MPPs. Thus, during operation, each of n-1 DC-DC convertingunits 506 may only need to process a small amount or percentage of thepower generated by the corresponding PV panel 504, since only a smalladjustment to its generated PV panel current may be required to make itsubstantially equal to I_(pv) series current or be within apredetermined threshold range of I_(pv) series current while thecorresponding PV panel 504 is operated at or near its target MPP value.As stated above, for similarly manufactured and sized PV panels 504exposed to substantially identical sunlight conditions, their individualMPPs may be substantially similar, which may lead to a substantiallysmall MPP variance throughout PV panel string 503.

Based on the PV string configuration of FIG. 5, during operation, if oneof the n-1 coupled PV panels 504, say for example PV₂ 504, is determinedto be producing a low current, i.e., I_(pv2), compared to the I_(pv)series current, because of shading conditions for example, controllingunit 604 determines whether a difference between the I_(pv) seriescurrent and the low current I_(pv2) is greater than a predeterminedcurrent difference threshold. In case, the determined difference isgreater than the predetermined current difference threshold, thencontrolling unit 604 is configured to trigger the correspondingconverting unit DC-DC₁ 506 to shunt PV panel PV₂ 504 and perform as abypass unit to the I_(pv3) current, which is output by PV panel PV₃ 504,and provide it as an input current to PV panel PV₁ 504, therebydelivering the I_(pv3) current around shunted low-performing PV panelPV₂ 504. In one embodiment, the predetermined current differencethreshold may be about 10% of the I_(pv) series current. Alternatively,any other value for the predetermined current difference threshold maybe chosen. During this bypass activity, controlling unit 604 may also beconfigured to trigger converting unit DC-DC₁ 506 to direct the lowcurrent I_(pv2) to electrical node 512 where it may be added to theI_(pv) series current.

Alternatively, in lieu of using controlling unit 606, the bypassactivity (PV panel shunting) may be triggered by a DC-DC converting unit506 when it determines that the correspondingly coupled PV panel 504generates a current that is less than a predetermined current differencethreshold of currents generated by adjacent PV panels 504.

As it would be obvious to one of ordinary skill in the art, theconfiguration of FIG. 5 may be extended to an individual PV cell level,in which case each PV cell may be coupled to a differential powerprocessing system or circuitry, and no reverse diodes, as taught in theprior art, may be required. That is, the MPP Tracking process algorithmmay be implemented locally at the level of a PV element, which may be aPV cell, a string of PV cells, a PV panel, or a string of PV panels.

Based on the above discussion about DC-DC converting units 506, thedifferential power processing provides at least the following features:

Since DC-DC converting units 506 process only a small fraction of thetotal power generated by PV string 503, power losses are substantiallylow in comparison to DC-DC converting units that are required to convertall of the power generated by their respective PV panels 504.

Sizes of DC-DC converting units 506 can be relatively small given theunits limited power ratings requirements.

Costs of these DC-DC converting units 506 can be substantially low giventheir smaller sizes and limited power ratings.

Dynamic requirements and protection needs can be substantially low sinceeach DC-DC converting unit 506 is configured to manage only smallcurrent differences.

Now referring to FIG. 6, an exemplary embodiment 600 of a computingsystem 602 for controlling and optimizing power generated by nseries-connected PV panels 504 is shown. As above, computing system 602includes controlling unit 604 coupled to n-1 DC-DC converting units 506and to AC-DC inverting unit 508. Each of n-1 DC-DC converting units 506is configured to make each respective PV panel 504 operate at its MPP,which corresponds to the operating condition that facilitates capture ofthe most energy, and thereby generation of the most power. This isaccomplished via MPP tracker units 612, each of which may be an integralelement of one of n-1 DC-DC converting units 506. Alternately, MPPtracker units 612 may be stand-alone units coupled to one or more of then-1 DC-DC converting units 506. As shown, each DC-DC converting unit 506may include a processing unit 613 and a memory unit 614, both incommunication with corresponding MPP tracker unit 612.

Controller unit 604 includes a processing unit 606 and a memory unit608, which in turn includes a differential power program or application610. Processor unit 606 is configured to execute instructions and tocarry out operations associated with computing system 602. For example,using instructions retrieved from memory unit 608, processor unit 606may control the reception and manipulation of input and output databetween components of computing system 602. Various architectures can beused for processor unit 606, including dedicated or embedded processoror microprocessor (μP), single purpose processor, controller or amicrocontroller (μC), application-specific integrated circuit (ASIC),any combination thereof, and so forth. In most cases, processor unit 606together with an operating system operates to execute computer code andproduce and use data.

Memory unit 608 generally provides a place to store computer code anddata that are used by computing system 602. Memory unit 608 may includebut not limited to non-volatile memory, such as read-only memory (ROM,flash memory, etc.), volatile memory, such as random-access memory(RAM), a hard disk drive and/or the like. As stated above, memory unit608 includes differential power program or application 610, which isconfigured to monitor currents generated by the n PV panels 504, todetermine the I_(pv) series current, and to trigger the n-1 DC-DCconverting units 506 to operate their respective n-1 PV panels 504 attheir MPP values to achieve global MPP tracking. Alternatively, analogMPP trackers and analog DC-DC converting units, as well as analog DC-ACinverting units may be implemented for the process of maximizing powerconversion, via either local MPP tracking or a global MPP tracking.

Now referring to FIG. 7, an exemplary embodiment 700 of a powerconversion system 702, including n PV panels 704 connected in series andn-1 DC-DC converting units 706 connected in parallel, is shown.Similarly to the system configuration of FIG. 5, during operation as thebulk of the power, generated by series-connected PV panels 704, flowsdirectly through PV panels 704, only small power differences are made toflow through DC-DC converting units 706 via controlling unit 704, inorder to mitigate current mismatches between the generated PV panelcurrents, thereby substantially matching each of the generated PV panelcurrents with the determined I_(pv) series current. Further, upondetermination by controlling unit 604 that a current difference betweena low current generated by one of n-1 PV panels 704, coupled to one ofn-1 DC-DC converting units 706, and the I_(pv) series current is greaterthan the predetermined current difference threshold, then thecorresponding DC-DC converting unit 706 is configured to perform as abypass unit to the I_(pv) series current output by the preceding PVpanel 704 and provide it as an input current to the subsequent PV panel704, thereby delivering the I_(pv) series current around thelow-performing PV panel 704. During this bypass activity, controllingunit 604 may also be configured to trigger the corresponding DC-DCconverting unit 706 to direct the low generated current to an electricalnode where it may be added to the I_(pv) series current.

Now referring to FIG. 8A, an exemplary embodiment 800A of a powerconversion system 802, including n PV panels 804 connected in series andn-1 DC-DC converting units 806 connected in parallel, is shown. In thisarrangement of FIG. 8A, power conversion system 802 includes a DC poweror virtual bus 812. In this embodiment, each of n-1 DC-DC convertingunits 806 may send a portion of their respective power, diverted fromtheir corresponding PV panels 804, to an energy storage unit 812,hereafter referred to as virtual bus 812, rather than to a main DC bus(not shown) that may provide input to DC-AC inverting unit 808. Virtualbus 812 may be used to support control, communication, and otherfunctions, associated with power conversion system 802, that may need alow-voltage or separate DC power supply.

Now referring to FIG. 8B, another exemplary embodiment 800B of powerconversion system 820 includes n DC-DC converting units 822 connected inparallel, instead of only n-1 DC-DC converting units 806 as in powerconversion system 802. Similarly to the configuration of FIG. 8A, nDC-DC converting units 822 may send a portion of their respective powersto a virtual bus 824 rather than to the main DC bus (not shown)associated with AC-DC inverting unit 828. As stated above in relation tovirtual bus 812, virtual bus 824 may be configured to support control,communication, and other functions, associated with power conversionsystem 820, that may need low-voltage or separate DC power supply. Forexample, in case virtual bus 824 is a capacitive element, if duringoperation its voltage V_(c) is decreasing then it may be an indicationthat at least one of DC-DC converting units 822 may be drawing powerfrom virtual bus 824 in order to be able to maintain a tracking of thepredetermined MPP of the corresponding PV panel 826. Alternately, if thevoltage V_(c) of virtual bus 824 is increasing, then it may be anindication that one or more of DC-DC converting units 822 is providingpower to virtual bus 824 while maintaining an on-going tracking of thepredetermined MPP of the corresponding PV panel 826. Moreover, in thisarrangement of FIG. 8B, the monitoring and controlling of current andpower generated by individual PV panels 826 may be performed bycorresponding DC-DC converting units 822, rather than centrally by acentral control unit, such as control unit 604 of FIG. 6. That is, eachof DC-DC converting units 822 may be performing local MPP trackingindependently of a central control unit.

Now referring to FIG. 9, an exemplary embodiment 900 of a powerconversion system 902 including n PV panels 904, connected in series,and n-1 DC-DC converting units 906, with each of the DC-DC convertingunits 906 being connected to a couple of PV panels 904. In thisembodiment, each of n-1 DC-DC converting units 906 is also configured toperform as a power exchange unit between the correspondingly connectedcouple of PV panels 904. As such, in this FIG. 9 system configuration,any PV panel differential power can be exchanged entirely among PVpanels 904 as there is no second power flow circuitry, i.e., nosecondary power path, connecting n-1 DC-DC converting units 906 to AC-DCinverting unit 908. This differential power exchange enables PV panels904 to be adjusted, physically with respect to incident light, in realtime in order to adjust and substantially match their respective MPPrequirements. Moreover, because there is no second power flow circuitry,the value of the I_(pv) series current is determined so as to make a netsecondary power equal to zero, thereby automatically making the powerflowing through n-1 DC-DC converting units 906 as low as possible,thereby maximizing power conversion efficiency. Due to the zero value ofthe secondary current, power conversion system 902 is thus aself-contained power conversion system. In this embodiment, each of theDC-DC converting units 906 is either coupled to a pair of adjacent PVpanels 904 or to a pair of non-adjacent PV panels 904. As such, thecouple of PV panels 904 connected to an individual DC-DC converting unit906 need not be adjacent to one another. Moreover, in this arrangementof FIG. 9, the monitoring and controlling of current and power generatedby individual PV panels 904 may be performed by corresponding DC-DCconverting units 906, rather than centrally by a central control unit,such as control unit 604 of FIG. 6. That is, each of DC-DC convertingunits 906 may be performing local MPP tracking independently of acentral control unit.

FIG. 10 illustrates an exemplary embodiment 1000 of power conversionsystem 1002, which is configured, to provide differential powertransfers between neighboring PV panels 1004. To highlight aspects ofthe herein-introduced differential power processing, an exemplaryoperation of power conversion system 1002 is examined using a buck-boosttopology for the DC-DC converting units 1006. As shown, power conversionsystem 1002 includes four PV panels PV₁-PV₄ 1004 and three differentialDC-DC converting units DC-DC₂ to DC-DC₄ 1006. Moreover, any two adjacentDC-DC converting units 1006 are coupled to a common PV panel 1004. Forexample, converting units DC-DC₁ and DC-DC₂ are both coupled to PV panelPV₂ 1004, while converting units DC-DC₂ and DC-DC₃ are both coupled toPV panel PV₃ 1004. As illustrated, each differential DC-DC convertingunit 1006 includes an inductive storage element L 1007, and a couple ofswitching elements 1009. Global MPPT 1010 is also shown.

In power conversion system 1002, panel PV₃ 1004 may be subject to ashading condition while PV panels PV₁, PV₂ and PV₄ 1004 may not besubject to a similar shading condition. Assuming all other physical andelectrical aspects of each of PV panels PV₁-PV₄ 1004 are substantiallysimilar, based on operation data received or collected from PV panelsPV₁-PV₄ 1004, assume their corresponding voltages, attained whileoperating at their respective MPPs, are V_(mpp1)=V_(mpp2)=V_(mpp4)=37.09volts (V) and V_(mpp3)=36.80 V. The duty ratios of DC-DC convertingunits 906 are controlled, via controlling unit 604, to regulate PV panelvoltages of PV₂, PV₃, and PV₄ 10004 at the respective local MPPs. Forthe sake of simplicity, the MPP for PV₁ can be represented as beingaccomplished through a variable resistor (not shown) whose value isselected to maximize a global power output (in this case, R=29.8 Ohms)and, in effect, meet the local MPP of PV panel PV₁ 1004. The currentsflowing in power conversion system 1002 flow in such a way that each ofPV panels PV₁-PV₄ 1004 provides its MPP current, as follows:I_(mpp1)=I_(mpp2)=I_(mpp4)=5.09 amperes (A) and I_(mpp3)=4.58 A. Aftersome analysis, it can be shown that the currents in the differentialDC-DC converting units 1006 are I_(L1)=0.2535 A, I_(L2)=0.509 A, andI_(L3)=−0.2545 A. These three current values enable each of PV panelPV₁-PV₄ 1004 to operate at their respective MPPs.

Additionally, the here-in introduced differential power processing canlead a substantial simplification of the global MPP tracking since localPV panel minima are eliminated, providing each DC-DC converting unit1006 is able to meet the local MPP condition and that there are no localminima for the respective PV panels 1004.

Now referring to FIG. 11, a schematic diagram illustrates an exemplaryembodiment 1100 of a local controller system 1114, associated with orembodied in a differential DC-DC converting unit 1106 for providingpower conversion monitoring and control in differential power conversionsystem 1102. As shown, local controller system 1114 includes acontroller or controlling unit 1116 and an MPPT application or algorithm1118. A selection of MPP tracking algorithms 1118 is not constrained bythis differential power approach, i.e., any MPPT algorithm may befeasible, although some may be more effective. For example, if DC-DC₁differential converting unit 1106 is implemented on a sub-module basiswhere a control overhead (energy that controller system 1114 consumes)may need to be small, substantially simple MPPT algorithms may be used.During operation, once voltage V_(PV1) and current I_(PV1) of panel PV₁are measured, MPPT algorithm 1118 may use this measurement informationto generate an operating reference for controlling unit 1116. Thisgenerated operating reference may be a current reference, a voltagereference, a duty ratio, etc. . . . , depending on the controller type.Controlling unit 1116 is configured to generate switching signals forswitching elements q₁ and q₂ 1109, such as through a pulse widthmodulation technique, although pulse frequency modulation technique andother suitable techniques may also be feasible. The switching signalscontrol the turn-on/turn-off action of the switches q₁ and q₂ 1109 indifferential DC-DC converting unit 1106.

Moreover, in the above-discussed embodiments, the local control of PVpanels, via their respective DC-DC converting units, enables distributeddiagnostics and monitoring of these PV panels. This local controlarrangement can help detect defective PV panels, which may lead toreducing time to repair, and improve understanding of the operationconditions of the power conversion system. The differential powerconverting units may also shunt (directly bypass by a switching element)a failed or defective PV panel, or a PV panel whose MPP current isdetermined to be too low with respect to the determined I_(pv) seriescurrent. Furthermore, as discussed above, each of differential powerconverting units may also be configured to open the PV panel string andstop the flow of current and power for safety reasons or trigger ashunt-based string-level protection. Thus, in the above-discussedembodiments, an overall reliability of the power conversion system isimproved since a failure in a PV panel or in a differential powerconverting unit need not cause more extensive system failure or generalsystem failure.

Now referring to FIG. 12, a flow chart 1200 illustrating an embodimentof a method for controlling power generated by n series-connected PVpanels associated with any of the above-discussed converting unitarrangements. At step 1202, controller unit 604 is configured to monitorand compare a plurality of currents generated the n series-connectedsolar power units. Based on the comparison, controller unit 604 isconfigured to determine a series current for the n series-connectedsolar power units, at step 1204. Upon determination of the seriescurrent, the n series-connected PV panels are controlled via the n-1power converting units and the power inverting unit, so that each of thegenerated photovoltaic currents is substantially equal to the determinedseries current, at step 1206. Controller unit 604 is further configuredto determine whether any of the generated photovoltaic currents is lessthan a predetermined current threshold percentage of the determinedseries current, at step 1208. If it is determined that one or more ofthe generated photovoltaic currents is less than a predetermined currentthreshold percentage, then controller unit 604 is further configured tocause the corresponding DC-DC converting units to operate as currentbypass units, at step 1210.

The above discussed embodiments of power conversion systems enable thefollowing:

A power production from individual PV panels is substantially maximizedunder realistic environmental conditions.

An actual energy production is improved because of local and global MPPoperations.

An incremental cost for any of the above discussed embodiments issubstantially less than that of prior-art panel-based converting units.

DC-DC converting units needed to implement the above-discussed methodmay not need to have high performance characteristics to produce systembenefits.

Since only fractional power is processed by the DC-DC converting units,an implementation of the above-discussed power conversion systems lendsitself to subpanel and even cell-level power processing.

While a power managed by differential converting units may increase asenvironmental conditions vary more widely, as with shading, benefits arestill possible even with limited action. For example, under partialshading, any differential power management improves production. Inaddition, differential conditions affecting PV panels may be typicallytemporary, so the converting units involved may not need to processsubstantial power levels for more than a few minutes—unless a cell isdamaged or soiled. In the case of severe soiling that may compromiseperformance of a PV panel or PV cell for a long time, the associatedexchange converting unit may simply bypass the PV panel or PV cellentirely, thus avoiding the need to actually process any associatedpower. These above-discussed power conversion systems can be applied tolarge PV systems with many series-connected PV panels, or as a basis fora new class of cell-by-cell PV power processing.

In some embodiments, the disclosed methods may be implemented ascomputer program instructions encoded on a non-transitorycomputer-readable storage media in a machine-readable format. FIG. 13 isa schematic 1300 illustrating a conceptual partial view of an examplecomputer program product 1302 that includes a computer program forexecuting a computer process on a computing device, arranged accordingto at least some embodiments presented herein. In one embodiment, theexample computer program product 1302 is provided using a signal bearingmedium 1303. The signal bearing medium 1303 may include one or moreprogramming instructions 1304 that, when executed by one or moreprocessors may provide functionality or portions of the functionalitydescribed above with respect to FIG. 12. Thus, for example, referringthe embodiment shown in FIG. 12, one or more features of blocks 1202,1204, 1206, 1208, and/or 1210 may be undertaken by one or moreinstructions associated with the signal bearing medium 1303.

In some examples, the signal bearing medium 1303 may encompass anon-transitory computer-readable medium 1305, such as, but not limitedto, a hard disk drive, a Compact Disc (CD), a Digital Video Disk (DVD),a digital tape, memory, etc. In some implementations, the signal bearingmedium 1303 may encompass a computer recordable medium 1306, such as,but not limited to, memory, read/write (R/W) CDs, RJWDVDs, etc. In someimplementations, the signal bearing medium 1303 may encompass acommunications medium 1307, such as, but not limited to, a digitaland/or an analog communication medium (e.g., a fiber optic cable, awaveguide, a wired communications link, a wireless communication link,etc.). Thus, for example, the signal bearing medium 1303 may be conveyedby a wireless form of the communications medium 1007 (e.g., a wirelesscommunications medium conforming with the IEEE 802 standards or othertransmission protocols).

While various aspects and embodiments have been disclosed herein, otheraspects and embodiments will be apparent to those skilled in the art.The various aspects and embodiments disclosed herein are for purposes ofillustration and are not intended to be limiting, with the true scopeand spirit being indicated by the following claims, along with the fullscope of equivalents to which such claims are entitled. It is also to beunderstood that the terminology used herein is for the purpose ofdescribing particular embodiments only, and is not intended to belimiting.

What is claimed is:
 1. A solar power system for optimizing solar powerconversion, comprising: a set of n solar power units connected inseries, wherein each of the n solar power units generates a DCphotovoltaic current and produces a DC photovoltaic voltage; a set ofn-1 DC-DC converting units, each of the n-1 DC-DC converting units iscoupled to at least one of the n solar power units, and wherein each ofthe n-1 DC-DC converting units is configured to control thecorrespondingly connected solar power units to operate at a targetcurrent generation; a DC-AC inverting unit for inverting a cumulative DCphotovoltaic voltage of the n produced photovoltaic voltages to an ACpower signal for distribution to one or more AC loads, wherein the DC-ACinverting unit is configured to control operation of any one of the nsolar power units, which is not coupled to one of the n-1 DC-DCconverting units; and a controlling unit coupled to the n-1 DC-DCconverting units and to the DC-AC inverting unit, wherein thecontrolling unit monitors and compares the n currents generated by the nsolar power units, wherein based on the current comparison thecontrolling unit determines a series current and controls the n solarpower units, via the n-1 DC-DC converting units and the DC-AC invertingunit, so that each of the generated photovoltaic currents issubstantially equal to the determined series current, and wherein whenthe controlling unit determines that one of the n-1 solar power units,coupled to one of the n-1 DC-DC converting units, generates aphotovoltaic current that is less than a predetermined current thresholdpercentage of the determined series current, the controlling unit causesthe correspondingly coupled DC-DC converting unit to operate as acurrent bypass unit.
 2. The solar power system of claim 1, wherein eachof the n currents is generated based on a corresponding MPP trackingdetermined by the correspondingly coupled DC-DC converting unit.
 3. Thesolar power system of claim 1, wherein each of the n currents isgenerated based on a global MPP tracking determined by the DC-ACinverting unit.
 4. The solar power system of claim 1, wherein the n-1DC-DC converting units are connected in series or in parallel.
 5. Thesolar power system of claim 1, wherein each of the n-1 DC-DC convertingunits is connected to two solar power units.
 6. The solar power systemof claim 1, wherein the predetermined current threshold percentage ofthe determined series current is about ninety percent.
 7. The solarpower system of claim 1, further comprising a first DC bus connected tothe DC-AC inverting unit, and a second DC bus connected to each of then-1 parallel connected DC-DC converting units.
 8. A solar power systemfor optimizing solar power conversion, comprising: a plurality of solarpower units connected in series, wherein each of the plurality solarpower units generates a DC photovoltaic current and produces a DCphotovoltaic voltage; and a plurality of DC-DC converting units, each ofthe plurality DC-DC converting units is coupled to at least one of theplurality of solar power units, wherein each of the plurality of DC-DCconverting units is configured to control operation of thecorrespondingly connected solar power units, wherein an individual DC-DCconverting unit is configured to determine whether a correspondinglyconnected solar power unit generates a corresponding DC photovoltaiccurrent that is less than a predetermined threshold percentage of atarget series current, and based on the determination that thecorresponding DC photovoltaic current is less than the predeterminedthreshold percentage of the target series current, the individual DC-DCconverting unit is configured to shunt the correspondingly connectedsolar power unit while the other solar power units remain connected inseries.
 9. The solar power system of claim 8, wherein the generatedcorresponding DC photovoltaic current is less than a predeterminedthreshold percentage of the target series current because of a defectiveoperating state of the correspondingly connected power unit.
 10. Thesolar power system of claim 8, wherein the generated corresponding DCphotovoltaic current is less than a predetermined threshold percentageof the target series current because of a shading of the correspondinglyconnected power unit.
 11. The solar power system of claim 8, wherein theplurality of DC-DC converting units are connected in series or inparallel.
 12. The solar power system of claim 8, wherein each of theplurality of DC-DC converting units is connected to two solar powerunits.
 13. The solar power system of claim 8, further comprising a firstDC bus connected to the DC-AC inverting unit, and a second DC busconnected to each of the plurality of DC-DC converting units.
 14. Thesolar power system of claim 8, wherein the predetermined currentthreshold percentage of the determined series current is about ninetypercent.
 15. A solar power system for optimizing solar power conversion,comprising: a plurality of solar power units connected in series,wherein each of the plurality of solar power units generates a DCphotovoltaic current and produces a DC photovoltaic voltage; a pluralityof DC-DC converting units, each of the plurality of DC-DC convertingunits is coupled to at least one of the plurality of solar power units,wherein each of the plurality of DC-DC converting units is configured tocontrol the correspondingly connected solar power units so that theyeach generate at a corresponding target current; and a controlling unitcoupled to the plurality of DC-DC converting units, wherein thecontrolling unit monitors and compares the plurality of currentsgenerated by the plurality of solar power units, wherein based on thecurrent comparison the controlling unit determines a series current andcontrols the plurality of solar power units, via the plurality of DC-DCconverting units, so that each of the generated photovoltaic currents issubstantially equal to the determined series current, wherein, each ofthe plurality of DC-DC converting units is configured to determine anoperating status of each of the correspondingly connected solar powerunits, and to report the operating status to the controlling unit. 16.The solar power system of claim 15, wherein reporting the operatingstatus of an individual correspondingly connected solar power unitcomprises reporting whether the individual correspondingly connectedsolar power unit is in a defective state.
 17. The solar power system ofclaim 15, wherein the plurality of DC-DC converting units are connectedin series or in parallel.
 18. The solar power system of claim 15,wherein each of the plurality of DC-DC converting units is connected totwo solar power units.
 19. The solar power system of claim 15, furthercomprising a first DC bus connected to the DC-AC inverting unit, and asecond DC bus connected to each of the parallel connected DC-DCconverting units.
 20. A solar power system for optimizing solar powerconversion, comprising: a plurality of solar power units connected inseries, wherein each of the plurality of solar power units generates aDC photovoltaic current and produces a DC photovoltaic voltage; aplurality of DC-DC converting units, each of the plurality of DC-DCconverting units is coupled to at least one of the plurality of solarpower units, wherein each of the plurality of solar power units iscoupled to two of the plurality of DC-DC converting units; and acontrolling unit coupled to the plurality of DC-DC converting units,wherein the controlling unit monitors and compares the plurality ofcurrents generated by the plurality of solar power units, wherein basedon the current comparison the controlling unit determines a seriescurrent and controls the plurality of solar power units, via theplurality of DC-DC converting units, so that each of the generatedphotovoltaic currents is substantially equal to the determined seriescurrent.